There is considered application of a nitride semiconductor to a semiconductor device with high withstand voltage and high output power, utilizing characteristics such as high saturation electron speed and wide band gap. For example, the band gap of GaN as the nitride semiconductor is 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV), and thus GaN has high breakdown electric field intensity. Accordingly, GaN is quite promising as a material of a semiconductor device for power supply which obtains high voltage operation and high output power.
As a semiconductor device using the nitride semiconductor, there have been made numerous reports on a field effect transistor, particularly a high electron mobility transistor (HEMT). For example, among GaN-based HEMTs (GaN-HEMTs), AlGaN/GaN.HEMT using GaN as an electron transit layer and AlGaN as an electron supply layer is attracting attention. In the AlGaN/GaN.HEMT, a strain resulted from a lattice constant difference between GaN and AlGaN occurs in AlGaN. Two-dimensional electron gas (2DEG) of high concentration is obtained from piezoelectric polarization and spontaneous polarization of AlGaN caused by the strain. Accordingly, the AlGaN/GaN. HEMT is expected as a high efficiency switch element and a high-withstand-voltage electric power device for electric vehicle, or the like.    Patent Document 1: Japanese Laid-open Patent Publication No. 2009-76845    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-19309    Patent Document 3: Japanese Laid-open Patent Publication No. 2010-225765    Patent Document 4: Japanese Laid-open Patent Publication No. 2009-71061
Generally, a power switching element is required to perform a so-called normally-off operation in which no current flows through the power switching element when the gate voltage is 0 V. However, the GaN-HEMT has a problem of being difficult to realize a normally-off type transistor because of generation 2DEG of high concentration. To address the problem, studies have been conducted to decrease the concentration of 2DEG by etching an electron supply layer directly under a gate electrode so as to achieve Normally Off (see Patent Document 1). In this method, however, the etching damages the vicinity of an electron transit layer located under the electron supply layer, bringing about problems such as increase in sheet resistance, increase in leak current and so on. Hence, a technique has been proposed which additionally forms a p-type GaN layer having a p-type conductivity type between the gate electrode and an active region in the AlGaN/GaN.HEMT to thereby cancel the 2DEG directly under the gate electrode to achieve Normally Off (see Patent Document 2).
A schematic structure of the AlGaN/GaN.HEMT according to the prior art is exemplified in FIG. 1.
In this AlGaN/GaN.HEMT, a nucleus formation layer is formed on a substrate, an electron transit layer 101 composed of i (intentionally undoped)-GaN is formed thereon, and an electron supply layer 102 composed of i-AlGaN is formed thereon. 2DEG is generated in the vicinity of an interface of the electron transit layer 101 with the electron supply layer 102. A p-type GaN layer 103 is formed on the electron supply layer 102, and a gate electrode 104 is formed thereon. A source electrode 105 and a drain electrode 106 are formed on both sides of the gate electrode 104 on the electron supply layer 102.
At the time when no voltage is applied to the gate electrode 104, halls are unevenly distributed at a lower part of the p-type GaN layer 103 (in the vicinity of an interface of the p-type GaN layer 103 with the electrode supply layer 102). Electrons are drawn by the halls and induced in the vicinity of the interface of the electron transit layer 101 with the electron supply layer 102 below the halls. This turns on a gate voltage Vg. Thus, there is a problem in which Normally Off is inhibited to fail to increase a threshold voltage.